Security forces fired tear gas into the sit-in in, and live images of Rabaa al-Adawiya showed smoke billowing from the square and military helicopters flying overhead.

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Canisters of tear gas rained down on tents set up by the protesters at one end of the Rabaa al-Adawiya camp as police vehicles, one blaring a siren, advanced on the protesters.

“It is the beginning of the operation to disperse the protesters,” a security official told AFP.

The Al Arabiya correspondent reported that security forces were opening up Nasr Street, a road which leads out of Rabaa al-Adawiya, for those wanting to leave camp.

The correspondent added that some protesters had begun burning tires to block the security forces efforts.

An Associated Press television video journalist at the scene of the larger of the two camps said he could hear the screams of women as a cloud of white smoke hung over the site in the eastern Cairo suburb of Nasr City.

He said an army bulldozer was removing mounds of sand bags and brick walls built by the protesters as a defense line in the Nasr City camp. Army troops, however, were not taking part in the operation.